A man died last night during the G20 protests in central London as a day that began peacefully ended with police saying bottles were thrown at police medics trying to help him.

The man had collapsed within a police cordon set up to contain the crowds who had assembled in central London and the City to protest over the G20 summit. There were 63 arrests on the day.

The Independent Police Complaints Commission was being notified last night. Scotland Yard said the alarm had been raised by a member of the public who spoke to a police officer on a cordon at the junction of Birchin Lane and Cornhill in the City.

He sent two medics through the cordon line and into nearby St Michael's Alley where they found a man who had stopped breathing. They called for ambulance support at about 7.30pm and moved him back behind the cordon where they gave him cardio-pulmonary resuscitation.

"The officers took the decision to move him as during this time a number of missiles – believed to be bottles – were being thrown at them", said a police statement. The ambulance service took the man to hospital where he died.

A London ambulance spokesman said: "Our staff immediately took over the treatment of the patient and made extensive efforts to resuscitate him both at the scene and on the way to hospital."

The directorate of public standards at both the Metropolitan and City of London police had been informed, the statement said. One protester at the scene said the man was in his 30s and died of natural causes, the Press Association news agency reported.

The man's death ended a day in which the contrasting faces of British policing were on display in London.

The Met called in support from 30 forces across the country to create a 5,000-strong team of officers for at least six diverse demonstrations in the City of London and Trafalgar Square. Outside the Bank of England police horses and riot officers were pushed back by the sheer force of demonstrators – helmets were torn from officers' heads and cans, fruit and flour rained down. In retaliation the police surged forward, cracking heads with batons, using pepper spray and CS gas, and sirens wailed all around.

Three minutes' walk away, in Bishopsgate, smiling officers shared a joke with men and women pitching tents along the road, a family offered them chocolate brownies from an organic food stall and a few lads politely queued up outside the compost toilet tent.

But late last night there was a stand-off as officers moved to start to break up the climate camp that had been set up.

Violence spread as far as London Bridge, with riot police chasing groups of demonstrators, who responded with bottles and other missiles.

Commanders at the Met, who are said to be among the best public order officers in the world, insisted they would not let the city be brought to a standstill.

They used familiar tactics to trap 4,000 people into streets outside the Bank of England in a practice known as "kettling", tightening the cordon when violence flared in one part of Threadneedle Street and a group of protesters, whose faces were covered, broke into the Royal Bank of Scotland.

Commander Bob Broadhurst, in charge of the operation, said his aim was to facilitate peaceful protest. But those peaceful demonstrators caught inside the cordon with no toilet facilities, and little water, questioned the idea that they were being allowed to exercise their right to march.

"The police should let us dribble out when we need to," said June Rogers, a gardener from south London. "We've just come on a peaceful protest. We've got fire in our belly and we want to say something and be heard, we are just ordinary people but they made the situation worse."

Jeannie Mackie, a barrister who had attended the climate camp as an observer, was penned in for two hours after police cordoned off both ends of Bishopsgate.

"I thought it was completely unnecessary," she said.

"I was kept for two hours. Lines of police lined up with their batons and they were completely pumped up and looking to have a go. My feeling was everyone in there was peaceful but they wanted to clear them out." Responding to the police use of the kettling technique she said that although the courts had ruled that it was legal, there had to be a good reason. "I asked one officer could I go and he said no – I might to and cause trouble. I giggled and said that wasn't very likely and he said, 'you can never tell with these people'."

Scotland Yard said a cordon was used because missiles were being thrown at officers. It also said that portaloos and water had been moved in.

Earlier in the day demonstrations had started close to the Bank of England, storming a Royal Bank of Scotland branch, and baton-wielding police charging a sit-down protest by students.

Much of the protesting was peaceful, but some bloody skirmishes broke out as police tried to keep thousands of people in containment pens surrounding the Bank of England on Threadneedle Street.

Some buildings in the City had been boarded up in anticipation of trouble, with staff warned to work from home or dress down.

As protesters began to gather, after 11am, some City workers were seen waving £10 notes at them from office windows.

After the charge against the sit-down protest at students, there were complaints that officers had been heavy handed.

"When people surrounded RBS, I could understand police tactics," said Jack Bright, 19. "We were sat down, trying to have a peaceful protest, but they started whacking us."